This invention relates to an electronic weight measuring device with a load cell.
Conventionally, there are known electronic weight measuring devices which are provided with a load cell to produce output signals corresponding to weight and an analog-digital (A/D) converter for converting the output signals from the load cell into digital signals. In one such electronic weight measuring device, the load cell produces a unipolar, e.g. negative voltage signal corresponding to a weight, and the A/D converter produces a digital output signal corresponding to the negative output voltage from the load cell. The digital output signal from the A/D converter is processed by a digital microprocessor, and then supplied to a display unit. Thus, in the prior art electronic weight measuring device, the load cell supplies the A/D converter with a negative output voltage within a range corresponding to a given weight measuring range, so that the A/D converter need be so designed that any input voltage varying within the range corresponding to the weight measuring range may be produced.
In order to eliminate such problem, there is used a zero point setting circuit producing an output voltage with a polarity, e.g. positive, opposite to that of the output voltage from the load cell. The output voltages from the zero point setting circuit and load cell are added, and an addition output voltage is supplied to an A/D converter capable of processing positive and negative input signals. For example, the load cell may be so constructed that the output voltage of the load cell is at zero level when no load is supplied thereto and at -V.sub.L level when the maximum rated load is applied thereto, and the output voltage of the zero point setting circuit may be set at +1/2 V.sub.L so that the A/D converter circuit may be supplied with an input voltage ranging from -1/2 V.sub.L to +1/2 V.sub.L. Hereupon, the A/D converter circuit includes an absolute value generator section to produce an output signal corresponding to the absolute value of an input voltage, and an A/D converter section to process an output signal from the absolute value generator section. Accordingly, an A/D converter capable of processing input voltages ranging from 0 to 1/2 V.sub.L can be used for the A/D converter section. Thus, such device has an advantage over the conventional device because the A/D converter circuit can process an input voltage with a level twice that of a permissible maximum input voltage to the A/D converter section.
The electronic weight measuring device of this type, including an A/D converter circuit for converting bipolar input voltages into digital signals, requires a sensitivity regulator circuit for changing the level of the output signal from the absolute value generator section to regulate the sensitivity of the A/D converter section. Suppose, for example, a case where the A/D converter section is adjusted to produce an output signal for a count of 20,000 when no load is applied and to produce an output signal for a count of -20,000 when the maximum allowable weight is applied. In this case, it is possible first to regulate the zero setting circuit in a no-load state so that the output signal from the A/D converter section may correspond to the count of 20,000, and then to adjust the sensitivity regulator circuit so that the A/D converter section may produce an output signal corresponding to the count of -20,000, thereby representing the range of weight measured between no-load and maximum allowable load states by a count of 40,000. However, although the adjustment of the sensitivity of the A/D converter section by means of the sensitivity regulator circuit causes such sensitivity to vary from the sensitivity obtained when the zero point regulator circuit is adjusted, the no-load input level of the A/D converter section, fixed by the zero point regulator circuit, will never change, so that the no-load count output of the A/D converter section will possibly change after completion of the adjustment. Accordingly, in order to obtain a count difference of 40,000 between the no-load and maximum allowable load states, the zero point regulator circuit and sensitivity regulator circuit need be adjusted repeatedly.
An object of this invention is to provide an electronic weight measuring device with an A/D converter capable of easily performing sensitivity regulation.